Adaptive optical system with self-referencing contrast control
A system for wavefront aberration reduction of an incident optical beam. The system includes a spatial light modulator (SLM) for receiving the incident optical beam and forming an output optical beam. The output beam is aberration-reduced compared to the incident beam. An interferometer receives a portion of the output optical beam and generates an interference fringe pattern by introducing a phase shift to one part of the portion of the output optical beam, the interference fringe pattern being applied to the SLM. The interference fringe pattern is representative of the wavefront error of the incident optical beam and the interference fringe pattern activates the SLM such that the SLM performs wavefront error correction on the output optical beam.
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This disclosure relates to the field of closed-loop adaptive optical systems, and in particular, to sensing and correcting wavefront errors.
BACKGROUND OF THE INVENTIONThe prior art includes closed-loop adaptive optical systems which use conventional adaptive optical approaches such as deformable mirrors, wavefront error sensors (WESs), drive electronics and processors with N servo-loops, where N equals the number of resolvable pixels to be controlled. The number of resolvable pixels to be controlled may be on the order of a few hundred to over ten thousand.
Another approach to closed-loop systems includes an optical scheme to replace the N hard-wired servo-loops, and exploits a spatial light modulator (SLM) in an all-optical closed-loop configuration (U.S. Pat. No. 5,046,824; D. M. Pepper). In this system, a local reference beam is required that is coherent with respect to an incoming aberrated beam. The local reference beam coherently combines with the input beam to form a spatial interference pattern that is applied to and thereby activates the SLM. The generation of the local reference beam is rather challenging and it thus complicates the system architecture. For example, the local reference beam may be generated by spatially filtering a part of the incoming beam, phase shifting this spatially filtered beam (for quadrature interference), and then recombining it interferometrically with the incident beam to form the necessary fringe pattern.
Another approach involves sampling a diffraction-limited sub-aperture of the incoming beam (possibly amplifying a single speckle), phase shifting it, and then interfering it with the remainder of the incoming aberrated beam.
Yet another approach is to phase-lock a local oscillator to the incoming distorted beam.
Yet another approach employs a radial shearing method, whereby a sub-aperture of large wavefront radius is used as the local reference. However, this approach is very inefficient in terms of processing the incoming photons and, in addition, limits the spatial frequency compensation capability of the adaptive optical system.
The above examples all suffer from a relatively low photon efficiency. Moreover, since a common path is not used for the interfering beams, the system is sensitive to vibrations, and further, a long coherence length source is needed for the aberrated beam with a coherence length greater than any path difference in the system.
An example of an adaptive optical closed-loop system of the prior art is shown in
The reader is directed to the following references for additional information regarding SLM's and this area of technology:
-
- (1) “Spatial Light Modulator Technology—Materials, devices and Applications”, edited by Uzi Efron, Marcel Dekker, Inc. publisher, pp 619–643, the disclosure of which is hereby incorporated wherein by reference;
- (2) “Single-pixel demonstration of innovative adaptive optics by use of a charge-transfer membrane light modulator”, by P. V. Mitchell et al., Optic Letters, vol. 18, no. 20, Oct. 15, 1993, pp 1748–1750, the disclosure of which is hereby incorporated wherein by reference; and
- (3) Characteristics of innovative adaptive-optics that use membrane-based spatial light modulators”, C. J. Gaeta et al., J. Opt. Soc. Am. A, Vol. 11, No. 2, February 1994, pp 880–894, the disclosure of which is hereby incorporated wherein by reference.
The object of the prior art system of
In addition to wavefront scrubbing, the system can also be used to generate a phase-conjugate replica 11 (a wavefront-reversed and aberration-reversed beam) of a readout beam 6. In this case, the same architecture may be used, but, in addition, a plane wave readout beam 6 is directed into the reverse direction of the scrubbed output beam 5, as shown in
In general, the adaptive optical system senses the wavefront distortion of the input beam 4 by sampling a portion of the external reference beam 8, using, for example, a beam splitter 7 preferably just after the SLM 1. This sampled external reference 8, which has some residual aberrations from region 14 since the scrubbing is not 100% effective, is then directed to the backside of the SLM (to the photoconductive input port 2), where it interferometrically combines with a coherent non-aberrated beam referred to as a local reference beam 9. The resultant interference pattern is an intensity mapping of the phase distortion of the external reference 8 relative to the local reference 9. Note that the local reference beam 9 is typically a plane wave. In general, upon convergence, the servo aspect of the adaptive optical system of
The local reference beam 9 may, in principle, be generated by beam splitting part of the external reference beam 8 and spatially filtering it using a conventional pinhole 20 with an amplitude stop of fixed diameter, as shown in
The performance of such prior art systems generally suffer from the following limitations:
-
- (1) A separate path is required for the generation of the local reference, which can lead to vibration-induced or thermally induced degradation of the system (the interferometric legs of the prior art systems must be maintained with a precision of a fraction of a wavelength (approximately λ/10) in path-length-differential to assure quadrature operation);
- (2) Since photons are lost in the spatial filtering operation of
FIG. 2 , that system is not photon efficient and suffers from significant losses; - (3) The added path length dictates the need for a laser or an optical source whose coherence length exceeds the path-length differences in the Mach-Zehnder interferometer (path length differences between the two interfering beams 8, 9); and
- (4) The fact that the pinhole embodiment (
FIG. 2 ) has a fixed diameter can lead to a degradation in performance. (It is to be noted that in accordance with one aspect of the present invention, the system may employ an amplitude pinhole with a variable diameter. In this case, the system has been shown, in simulations, to improve the convergence performance and dynamics (response time and Strehl ratio) of the closed-loop system.)
The prior art further teaches that a coherent local reference 9 may be generated by expanding (i.e., magnifying) part of the external reference beam 8 so that a fraction of the magnified wavefront is nearly planar (a portion of a spherical wave is nearly planar when the radius of the spherical wave becomes large as a result of magnifying the beam). This approach is discussed in the Jun. 1, 2000 issue of Optics Letters (Vol 25, No. 11).
However, when using this approach, the performance of the system is compromised for reasons which include the following:
-
- (1) Only a small fraction of the photons is utilized, resulting in a loss of performance;
- (2) Low order spatial frequencies are not processed, thereby limiting the spatial bandwidth of the system and resulting in a non-planar converged wavefront; and
- (3) Controlling the phase of the beams to realize quadrature of the external and local reference beams is not addressed in this approach. This limits the performance of the system as well as its robustness with respect to vibrations and other noise sources, as well as the need for a long coherence-length source, since the feedback-loop interferometer is not a common-path interferometer (as is utilized in the embodiments disclosed herein).
The present invention preferably uses a white-light interferometer (of the Zernike phase contrast class) to generate the interference pattern which activates the SLM for wavefront correction of the input beam. This white-light interferometer may have a fixed structure or may have a controllable phase-stop (see U.S. Pat. No. 4,833,314 to Pepper et al and U.S. Pat. No. 5,751,475 to Ishiwata et al., the disclosures of which are hereby incorporated herein by reference), and preferably uses a single control algorithm. The phrase “single control algorithm” is meant to indicate that one parameter can control the diameter of the phase-stop, as opposed to a set of controls, with each one sequentially opening or closing the phase-stop diameter, which is also taught by the Pepper patent.
In another embodiment, a simple control voltage can be used to control the phase-stop diameter as opposed to multiple voltage controls.
In either case, the overall closed-loop adaptive optics system can utilize a single control algorithm to vary the phase-stop diameter. However, multiple parametric control is also disclosed.
The present invention is more photon efficient than systems of the prior art as the disclosed feedback control loop utilizes most photons in the feedback beam, and thus is capable of accommodating sources with low input power levels as well as with poor spatial and spectral coherence (i.e., “white-light” sources with very short coherence lengths).
Further, in one embodiment of the white-light interferometer of the present invention, the phase stop of the system is variable (e.g., variable diameter) which optimizes the performance of the system, both temporally (convergence time) and spatially (Strehl ratio), as well as the phase shift of the phase stop itself (although it is believed that a phase-stop of 90° is likely optimum).
Applications of the present invention include but are not limited to:
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- laser communications systems (where diffraction-limited signal beams are required for optimal heterodyne or homodyne detection);
- beam clean-up for scaled-up laser systems, beam combining, optical interconnects, nonlinear optical mixing, applications where a plane wave is required for high efficiency wave mixing;
- compensated imaging systems (to view distant objects through turbulent atmospheres);
- remote sensing (including laser ultrasound sensing and enhanced vibrometry);
- optical fiber imaging and beam delivery systems (for multi-mode fibers);
Some of the advantages of the present invention over the prior art include, but are not limited to the following:
-
- An adaptive optical system can be realized without the need for complicated wavefront error sensors and associated computationally intensive reconstruction algorithms.
- The required coherent reference is preferably generated from the incident beam itself, and no additional coherent local reference beam is needed.
- The wavefront error sensing is accomplished interferometrically using a compact, common-path interferometer, which is robust, vibration tolerant, and capable of generating white-light fringe patterns (via the aberrated input beam).
- An optional single feedback controller is all that is required to provide the necessary control of over a large number (on the order of a million) of equivalent piston actuators. This optional feedback controller is preferably used if it is desired to control the characteristics (fringe visibility, wavefront quality, etc.) of the common-path interferometer as the system converges.
In accordance with the present invention, a novel white-light compatible interferometer is used in conjunction with a spatial light modulator (SLM), to form a robust adaptive optical compensation system. The disclosed white-light compatible interferometer may be used with lasers, but it may also be used with LEDs, spectral lamp sources and other classes of incoherent lamps and light sources having an effective coherence length only on the order of microns or greater.
In the system and method of the present invention, a portion of the input beam impinging on the system is diverted to the white-light interferometer for wavefront error sensing. From the diverted portion of the input beam, the white-light interferometer generates an interference fringe pattern representative of the wavefront distortions of the input beam, and the fringe pattern in turn activates the SLM which applies the appropriate wavefront corrections to the input beam.
In another aspect of the present invention, an optional feedback controller may be used to control the white-light compatible interferometer and optimize its performance.
Embodiments of the present invention with either Mach-Zehnder (or modified Mach-Zehnder) or common path interferometers are disclosed.
The combination of a white-light compatible interferometer and SLM, supplemented with an optional servo controller, form a compact and robust adaptive optical system, capable of providing wavefront correction of spectrally broadband sources (i.e., white light compensation). Further, an adaptive optical system with an equivalent million piston actuators can be realized using a single servo-control loop, with the number of equivalent piston actuators being dictated by the number of resolvable wavefront “coherent patches”, or spatial modes that characterize the wavefront distortion, as well as by signal-to-noise considerations (such as shot noise limitations, etc.).
In order to overcome some of the limitations of the prior art, embodiment of the present invention preferably employ a phase-stop module 30 (see, e.g.,
An embodiment of the present invention is shown in
In
The phase-stop module 30 preferably includes a phase-stop interferometric module or plate 31 having a first region 31a and a second region 31b, the first region 31a being a “stop region” and the second region 31b being a “pass region”. The phase-stop plate 31 is preferably positioned at a common focal plane of a pair of lenses 32, 33. Lenses 32, 33 preferably have identical focal lengths. A phase shift is imparted by the phase-stop interferometric module or plate 31 to a first portion of the external reference beam 8 which passes through the first region 31a thereof with the amount of the phase shift preferably being set to 90° relative to a second portion of the beam which passes through the second region 31b. The photons that pass through the stop region 31a of the phase-stop interferometric module 31 are of low spatial frequency content and can therefore be considered as being associated with a plane wave. Such photons (those passing through the stop region 31a) form the local reference beam. On the other hand, the photons that do not pass through the central phase-stop 31a, but propagate at greater diameters in the focal plane of the system, are of higher spatial frequencies and therefore pass through region 31b instead, and can thus be considered as being part of or reflecting the (aberrated) external reference, having a high spatial frequency content.
The phase-stop interferometric module 31 may be implemented in the form of, but is not limited to, a fixed diameter phase stop, as shown in the embodiment of
In contrast with the conventional amplitude spatial filter 20 discussed in connection with
-
- (1) The photon efficiency of the system is enhanced, since all or most of the photons incident upon the phase-stop interferometric module 31 are utilized in the generation of the desired fringe pattern for wavefront error sensing. This improves the signal-to-noise performance of the Wavefront Error Sensor (WES) (i.e., the phase-stop module 30) in terms of its shot-noise-limited detection of phase changes across the wavefront of the external reference beam. It follows that the system can properly function with lower incident optical intensities, thus enabling the system to compensate for weaker beams.
- (2) The fringe visibility of the interference pattern is preferbly optimized since the phase shift imparted by the phase-stop interferometric module 31 is set to 90°, and thus the local and external reference beams are in quadrature.
In addition, the phase-contrast interferometric system of the embodiment of the present invention of
-
- (3) The device is basically impervious to vibration, since both the local and external reference beams traverse the same basic path; and
- (4) Since the path lengths of the local and external reference beams are essentially equal, very spectrally broadband light (i.e., “white light”) can be used to generate the interference fringe pattern.
One of the possible control parameters for the system is the diameter of the stop region 31a of the phase-stop interferometric module 31. The diameter of the stop region 31a may be dynamically controlled using an optical sensor to preferably equalize the optical power passing through the phase stop region 31a and the optical power passing through the pass region 31b (i.e., preferably the same amount of optical energy passes through the first and second regions of the phase-stop interferometric module 31 to obtain more contrast in the fringe pattern). Dynamic control of the diameter of the stop region 31a adds complication (since a control system for controlling the aperture size of the stop region 31a is then needed), but since the benefits are believed to outweigh these complications, dynamic control is preferred for most applications. As will be seen, the embodiment of
Another way of dynamically controlling the aperture size of the stop region 31a is to maximize the fringe visibility of the fringe pattern that exits the phase-stop module 30 (fringe visibility is improved by increasing the contrast of the dark to bright fringes and this can be accomplished according to the embodiment to be discussed with reference to
In either embodiment (
Yet another possible control parameter is the magnitude of the phase-stop, and its adjustment around the optimal value of 90°.
With these two servo-control control parameters, i.e., the diameter of the phase stop and the magnitude of the phase shift, millions of equivalent piston actuators of the SLM 1 may be controlled.
An optional processor 40 may be used to control the diameter region 31a of the phase-stop and the magnitude of the phase shift imparted by the phase-stop module 30.
The embodiment of
An optical interference pattern is obtained in the plane of photoconductor input port 2 as a result of the phase-contrast operation of the phase-stop module 30. The interference pattern represents a near-field mapping of the wavefront residual error. As part of the closed-loop system, an optical relay module 43 is preferably used to image the small fraction of beam 89 transmitted by beam splitter 41 onto a video camera 44 having a CCD detector array or equivalent. The optical relay module 43 preferably comprises optical devices for relaying the beam such as lenses, mirrors, diffractive optics, etc., and may also include a magnification lens (to properly image and address the SLM) and/or a transform lens (to provide Strehl ratio information). The CCD array outputs an electrical signal representative of the fringe pattern, which signal is transmitted to processor 40 which may be implemented by a commercial video processor. Initially, the function of processor 40 is to ascertain the fringe visibility (i.e., the contrast level) of the optical interference pattern sensed by the video camera/CCD array 44. Another important function of processor 40 is to servo-control the phase-stop diameter of phase-stop (region 31a) interferometric module 31 in order to maximize the fringe contrast. In an initial search mode, the diameter of the phase-stop may first be set to a diffraction-limited value (approximately f#λ, where f# is the f stop and λ is the nominal wavelength) and then increased until the fringe contrast is maximized. As the closed-loop system begins to converge, the diameter of the phase-stop (region 31a) is preferably driven back to a value near its initial diffraction-limited diameter. As shown in
In another approach, the necessary optical measurements to be provided as inputs to processor 40, may be obtained with a dual-channel optical power detector 57, as shown in
There are several techniques by which this operation can be accomplished. In the embodiment of
Preferably, the secondary phase-stop module 50 includes a polarization beam splitter 54 following the polarization-stop interferometric element 51 which is preferably disposed between a pair of lenses 52, 53. The primary function of the polarization beam splitter 54 is to direct incident beams having different polarizations into different directions, and may be, for example, in the form of a Glan prism or another polarization beam splitter known on the art, for example, polymer-based devices and thin-film devices, etc. After passage through the polarization beam splitter 54, the first beam is preferably received by a first differencing detector 55, while the second is preferably received by a second differencing detector 56. The outputs of both differencing detectors are preferably directed to a dual-channel optical power detector 57, the output of which is sent to processor 40. The processor 40 determines the power difference between the first and second beams and in turn uses this information to control the phase-stop module 30 so that the power difference between the local and external reference beams generated by the phase-stop interferometric module 30 is kept at a minimal value, preferably near zero. In other words, the closed-loop system of
The embodiment of
Optionally, another camera/CCD imaging device 47 in combination with processor 40 can be used, either alternatively or in 10 combination with the improvements mentioned above with reference to
The embodiments of
Optionally, the output of the amplitude stop 22 can be sampled by a beam splitter 49 and a power measuring detector 48, in order to equalize the power in the near and far fields similar to the embodiments of
Optionally, a processor 40 can be utilized to control the aperture of reflecting surface 63a. The processor can use several pieces of information, such as (i) detection of the far field residual phase error using detector 44 and beam splitter 43; (ii) detection of the power of the second beam using detector 48n and beam splitter 49; or (iii) detection of the near field residual phase error using detector 47 (see
In yet another embodiment, both the phase-stop operation and the polarization tagging operation may be carried out with a single time-multiplexed interferometric module. Consider
The embodiments of
The embodiments of
All of these embodiments (of
Having described this invention in connection with a number of embodiments, modification will now certainly suggest itself to those skilled in the art. As such, the invention is not to be limited to the disclosed embodiments except as required by the appended claims.
Claims
1. A system for wavefront aberration reduction of an incident optical beam, the system including:
- a spatial light modulator for receiving the incident optical beam and forming an output optical beam, the output beam being aberration-reduced compared to the incident beam; and
- an interferometer for receiving a sampled portion of the output optical beam and for generating an interference fringe pattern by introducing a phase shift to one part of said sampled portion of the output optical beam, the interference fringe pattern being applied to said spatial light modulator and comprising essentially all of the light available in said sampled portion of the output optical beam;
- wherein the interference fringe pattern is representative of wavefront error of the incident optical beam and the interference fringe pattern activates said spatial light modulator such that said spatial light modulator performs wavefront error correction on the output optical beam.
2. The system of claim 1, wherein the interferometer comprises an interferometric module, said interferometric module including:
- (i) a stop region thereof for receiving a first portion of the sampled portion of the output optical beam; and
- (ii) a pass region thereof for receiving a second portion of the sampled portion of the output optical beam;
- wherein said first portion is changed by the stop region while said second portion passes through the pass region substantially unaffected.
3. The system of claim 2 wherein said first portion is changed by the stop region either (i) imparting a phase shift thereto; (ii) imparting a polarization change thereto; or (iii) imparting a direction change thereto.
4. The system of claim 2, wherein said first portion interferes with said second portion to form an interference fringe pattern.
5. The system of claim 2, wherein the spatial light modulator comprises (i) a photoconductor input port and (ii) a spatial phase output port adjacent the photoconductor input port, wherein the photoconductor input port receives the interference fringe pattern from the interferometer and addresses the spatial phase output port, and wherein the spatial phase output port performs wavefront error correction on the incident optical beam.
6. The system of claim 2, wherein the second optical beam is phase shifted by 90° relative to the second optical beam.
7. The system of claim 2 further comprising a beam splitter disposed in a light path of the output optical beam for sampling the output optical beam to form the sampled portion of the output beam by diverting said sampled portion of the output optical beam and transmitting a second portion of the output optical beam.
8. The system of claim 1 further including an opto-electric control apparatus for increasing a contrast of the interference fringe pattern.
9. The system of claim 8, wherein the opto-electric control apparatus comprises a polarization-stop interferometric module, said polarization-stop interferometric module including:
- (i) a stop region thereof for receiving a first portion of the sampled portion of the output optical beam; and
- (ii) a pass region thereof for receiving a second portion of the sampled portion of the output optical beam;
- wherein said first portion is imparted a polarization shift upon transmission by the stop region while said second portion passes through the pass region substantially unaffected.
10. The system of claim 8, wherein the interferometer comprises a phase-stop interferometric module, said phase-stop interferometric module including:
- (i) a stop region thereof for receiving a first portion of the sampled portion of the output optical beam; and
- (ii) a pass region thereof for receiving a second portion of the sampled portion of the output optical beam;
- wherein said first portion is imparted a phase shift upon transmission by the stop region while said second portion passes through pass region substantially unaffected.
11. The system of claim 10, wherein said first portion interferes with said second portion to form said interference fringe pattern.
12. The system of claim 10, wherein the spatial light modulator comprises (i) a photoconductor input port and (ii) a spatial phase output port adjacent the photoconductor input port, wherein the photoconductor input port receives the interference fringe pattern from the interferometer and addresses the spatial phase output port, and wherein the spatial phase output port perform wavefront error correction on the incident optical beam.
13. The system of claim 10, wherein said first portion is phase shifted by 90° relative to said second portion.
14. The system of claim 10 further comprising a beam splitter disposed in a light path of the output optical beam for sampling the output optical beam to form the sampled portion of the output beam by diverting said sampled portion of the output optical beam and transmitting a second portion of the output optical beam.
15. The system of claim 8, wherein the interferometer comprises a reflection-stop interferometric module, said reflection-stop interferometric module including:
- (i) a stop region thereof for receiving a first portion of the sampled portion of the output optical beam; and
- (ii) a pass region thereof for receiving a second portion of the sampled portion of the output optical beam;
- wherein said first portion is imparted a direction change upon reflection by the stop region while said second portion passes through the pass region substantially unaffected.
16. The system of claim 1 wherein the interferometer is a common path interferometer.
17. The system of claim 1 wherein the interferometer is a Mach-Zehnder interferometer.
18. The system of claim 1 wherein the interferometer is a quasi Mach-Zehnder interferometer.
19. The system of claim 1, wherein the interferometer comprises a reflection-stop interferometric module, said reflection-stop interferometric module including:
- (i) a stop region thereof for receiving a first portion of the sampled portion of the output optical beam; and
- (ii) a pass region thereof for receiving a second portion of the sampled portion of the output optical beam;
- wherein said first portion is imparted a direction change upon reflection by the stop region while said second portion passes through the pass region substantially unaffected.
20. A system for wavefront aberration reduction of an incident optical beam, the system including:
- a spatial light modulator for receiving the incident optical beam and forming an output optical beam, the output beam being aberration-reduced compared to the incident beam;
- an interferometer for receiving a portion of the output optical beam and for differentiating said portion into first and second beams, the interferometer having a dynamically controllable aperture;
- a phase shift apparatus for introducing a phase shift to one of said first and second beams which beams generate an interference fringe pattern which is applied to said spatial light modulator; and
- a control system for dynamically controlling the size of the aperture of the interferometer in order to reduce the amount of aberration in the output optical beam;
- wherein the interference fringe pattern is representative of the wavefront error of the incident optical beam and the interference fringe pattern activates said spatial light modulator such that said spatial light modulator performs wavefront error correction on the output optical beam.
21. The system of claim 20 wherein light passing through the aperture in the interferometer is phase-shifted relative to light passing the interferometer externally of said aperture.
22. The system of claim 20 wherein light passing through the aperture in the interferometer is polarization-shifted relative to light passing the interferometer externally of its aperture and further including a polarization beam splitter to separate the first and second beams, a phase shifter for imparting a phase shift to one of the first and second beams and a beam combiner for recombining the first and second beams after one of the first and second beams has been phase shifted by said phase shifter.
23. A system for wavefront aberration reduction of an incident optical beam, the system comprising:
- a spatial light modulator for receiving the incident optical beam and forming an output optical beam, the output beam being aberration-reduced compared to the incident beam;
- an interferometer for receiving a portion of the output optical beam and for differentiating said portion into first and second beams, the interferometer having a dynamically controllable aperture or reflective surface;
- a phase shift apparatus for introducing a phase shift to one of said first and second beams which beams generate an interference fringe pattern which is applied to said spatial light modulator; and
- a control system for dynamically controlling the size of the aperture or reflective surface of the interferometer in order to reduce the amount of aberration in the output optical beam;
- wherein the interference fringe pattern is representative of the wavefront error of the incident optical beam and the interference fringe pattern activates said spatial light modulator such that said spatial light modulator performs wavefront error correction on the output optical beam.
24. The system of claim 23 wherein the interferometer is a common path interferometer.
25. The system of claim 23 wherein the interferometer is a Mach-Zehnder interferometer.
26. The system of claim 23 wherein the interferometer is a quasi Mach-Zehnder interferometer.
27. A method for wavefront aberration reduction of an incident optical beam, the method including the steps of:
- receiving the incident optical beam at a spatial light modulator and forming an output optical beam, the output beam being aberration-reduced compared to the incident beam;
- directing a portion of the output optical beam to interferometer for differentiating said portion into first and second beams, said interferometer having a dynamically controllable aperture;
- introducing a phase shift to one of said first and second beams;
- generating an interference fringe pattern which is applied to said spatial light modulator; and
- dynamically controlling the size of the aperture of the interferometer to reduce the amount of aberration in the output optical beam;
- wherein the interference fringe pattern is representative of the wavefront error of the incident optical beam and the interference fringe pattern activates said spatial light modulator such that said spatial light modulator performs wavefront error correction on the output optical beam.
28. The method of claim 27 wherein light passing through the aperture in the common-path interferometer is phase-shifted relative to light passing the interferometer externally of said aperture.
29. The method of claim 27 wherein light passing through the aperture in the interferometer is polarization-shifted relative to light passing the interferometer externally of its aperture and further including the steps of separating the first and second beams, imparting a phase shift to one of the first and second beam, recombining the first and second beams after one of the first and second beams has been phase shifted, the recombined beams forming the interference fringe pattern.
30. A method for wavefront aberration reduction of an incident optical beam, the method including the steps of:
- receiving the incident optical beam at a spatial light modulator for and forming an output optical beam, the output beam being aberration-reduced compared to the incident beam;
- receiving a portion of the output optical beam at a interferometer for differentiating said portion into first and second beams, the interferometer having a dynamically controllable aperture or reflective surface;
- introducing a phase shift to one of said first and second beams which beams generate an interference fringe pattern which is applied to said spatial light modulator; and
- a control system for dynamically controlling the size of the aperture or reflective surface of the interferometer in order to reduce the amount of aberration in the output optical beam;
- wherein the interference fringe pattern is representative of the wavefront error of the incident optical beam and the interference fringe pattern activates said spatial light modulator such that said spatial light modulator performs wavefront error correction on the output optical beam.
4682025 | July 21, 1987 | Livingston et al. |
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Type: Grant
Filed: Dec 26, 2002
Date of Patent: Apr 11, 2006
Patent Publication Number: 20040125380
Assignee: HRL Laboratories, LLC (Malibu, CA)
Inventor: David M. Pepper (Malibu, CA)
Primary Examiner: Gregory J. Toatley
Assistant Examiner: Michael A. Lyons
Attorney: Ladas & Parry LLP
Application Number: 10/329,900
International Classification: G01B 9/02 (20060101);